LED lighting is becoming very important in the lighting industry. LED lighting has significant advantages over both incandescent and fluorescent lighting. LED lighting is more energy efficient than incandescent bulbs and LED lighting does not have the cold temperature use and mercury issues of the fluorescent bulbs. Furthermore, because of the small size of LED lights, LED lights can be packaged in ways that incandescent and fluorescent lighting cannot be packaged.
In the electronics industry, heat is a significant problem. The electrical devices in products, such as LED lighting, produce heat. This energy increases the temperature of the devices and of the system in which they are in. This, in turn, may reduce the performance and life of not only the devices themselves, but of the entire system. Therefore, one of the primary challenges in fully commercializing LED lighting is the solution to the thermal management of the heat generated by LED lighting in a cost effective manner. To date, most suppliers have used aluminum core circuit boards onto which they surface mount solder the LED device. Two dimensional aluminum core boards have limited surface area to dissipate heat. In addition, the LED lights cannot be easily interchanged to either replace defective units or to change the product color.
In order to reduce the effect of this detrimental energy, heat sinks are attached to the devices such as LED lighting. The heat sinks provide a means for removing the energy from the device through convection and radiation of the energy away from the device.
Energy loss from a heat sink occurs through natural convection, forced convection or radiation. The effectiveness of the heat sink in pulling energy away from the device is dependant on the ability to spread or dissipate the heat generated from what is often a small source over a larger area so that it can be removed through the flow of air over the surface or by radiation to the environment.
In effect, as long as the heat generated by the devices to be cooled can be effectively spread over a larger surface, the effectiveness of the heat sink is primarily dependent on the amount of available surface area. Whether the material is a conductor throughout its body or just on the surface does not affect its ability to transfer heat to the environment.
Heat management in electrical devices that are becoming smaller, lighter, and more compact is an ever increasing challenge. Historically, the heat sinks used to dissipate the energy have been made of metals such as zinc, aluminum, or copper and can be either machined, cast or extruded. Because the heat sinks are made of metal, the heat sinks are often heavy. As the devices become smaller and the need to reduce part weight and cost increases, alternative methods to control heat must be found. Furthermore, since the devices are electrical conductors, the attachment of heat sinks to the devices requires modifications to the heat sink so that electrical circuitry providing either signals or power can be provided without shorting such electrical circuitry to the metal heat sink.
Unlike incandescent and fluorescent lighting sources that perform well when they are warm/hot, LEDs need to be kept cool for optimal life and performance. By keeping the junction temperature (Tj) of the LED below its maximum temperature limit, the life of an LED can be extended to over 50,000 hours of operation. Because the heat generated at the LED junction is concentrated in a very small volume, the temperature can quickly rise to well over 150° C. if the heat is not efficiently removed.
The thermal model most commonly used for heat management at the LED junction between the LED device and the heat sink is provided by the following equation 1:Tj=Ta+Pd×Rja Where,                Tj is the junction temperature;        Ta is the ambient temperature for the heat sink;        Pd is the power that is being dissipated by the LED; and        Rja is the sum of the thermal resistances between the junction and ambient environment.        
The primary factors that affect the temperature at the junction Tj are:                1. The ambient temperature Ta of the LED device;        2. The sum of the thermal resistances between Tj and Ta;        3. The power that must be dissipated; and        4. The airflow.        
Assuming that the amount of power that must be dissipated by the LED is determined by the device being used, its efficiency, and the lighting requirements, the only variable in equation 1 that can be controlled is the thermal resistance between the junction and ambient Rja. The value of Rja can be determined by adding the various thermal resistances between the LED junction that emits the light and the ambient environment. A list of the resistance values that are included in Rja is as follows:                1. The thermal resistance of the LED die;        2. The thermal resistance of the die attach between the die and the internal heat sink;        3. The thermal resistance of the internal heat sink;        4. The thermal resistance between the internal heat sink and the solder point;        5. The thermal resistance of the solder pad on the substrate technology onto which the LED package is soldered;        6. The thermal resistance of the dielectric board onto which the LED package is mounted;        7. The thermal resistance of the attachment method between the dielectric board and a heat sink; and        8. The thermal resistance of the heat sink to the ambient environment.Of these thermal resistance values, items 1 through 4 are internal to the LED being manufactured by companies such as Lumileds, Cree, Osram, and Nichia. These values are predefined and can only be improved or modified by the manufacturers of the LEDs. The sum of these resistances is defined as Rjs, or the thermal resistance between the junction, j, and solder point, s, of the internal LED heat sink.        
Items 5 and 6 form the thermal resistance associated with the substrate technology chosen to both electrically and thermally interconnect the LED to the heat sink that will dissipate the heat to the ambient environment. This value is defined as Rsb where s is the solder point on the LED and b is the board onto which the LED is attached.
Items 7 and 8 are combined to create Rba where b is the board onto which the LED is mounted and a the ambient environment. In effect, Rba is the thermal resistance of the heat sink that is optimized for dissipating heat to the ambient environment.
When designing high power LED lighting devices, some of the guidelines provided by Cree (2) are as follows:                1. Reduce the amount of heat that must be removed near the LED junction by keeping the LED drive circuitry far enough away from the LED that it does not affect the junction temperature during operation.        2. Minimize the ambient temperature inside the fixture that encloses the LED by efficient thermal packaging. Optimizing heat dissipation surface area as well as ensuring that there is good natural or forced air flow will significantly improve thermal performance.        3. Minimizing the thermal resistance between the LED junction and ambient environment is critical to the success of keeping the junction temperature down. By eliminating or reducing the thermal resistances in the path between the two, it is possible to dramatically improve the performance of the heat management system.        4. The orientation of the LED/heat sink assembly is important in that some positions will enhance natural convection over heat dissipating surfaces and others will retard the flow of air.Of the guidelines provided above, the design of the thermal path between the LED junction and the surrounding ambient environment is what can be most affected for non-specific product designs. By either eliminating some of the thermal resistance paths or minimizing them, the temperature at the LED junction can be reduced.        
The following equation 2 is the result of adding the three grouped thermal resistances together to determine the value of Rja:Rja=Rjs+Rsb+Rba Examples of values of Rjs for some of the more common LEDs currently on the market are shown in the following table:
ThermalManufacturerLED TypeResistance (C/W)CreeXLamp XR (white, blue green)8CreeXLamp XR (amber, red)15OsramDiamond Dragon2.5OsramGolden Dragon (green)11LumiledsRebel10LumiledsK2 with TFFC5.5
The second value Rsb in equation 2 is the thermal resistance associated with the electrical substrate technology chosen. Because all high power LEDs have two electrical connections and a thermal connection, the LEDs must be attached to a substrate that provides electrical pats to a power source as well as a “thermal drain” into which heat can be pulled from the device. The two electrical connections being made to the LED are most often made on some form of circuit board material, but because all circuit board materials are made of some form of dielectric, there is a significant thermal resistance added to the thermal path between the LED and the ambient environment.
Testing performed by Osram on a variety of substrate technologies is summarized in Table 2.
Thermal ResistanceSubstrate TechnologyRsb (C/Watt)Metal Core PCB with enhanced dielectric3.4Metal Core PCB with FR4 dielectric7.3Flexible Printed Circuit on Al with standard9.5pressure sensitive adhesiveFlexible Printed Circuit on Al with thermal7.6enhanced pressure sensitive adhesiveFR4- PCB glued on Al with thermal vias9.7
The third component to the thermal resistance model, Rba, is the resistance between the board (i.e. substrate material) and the ambient environment. In most situations, this is the heat sink that is used to distribute the heat and transfer it to the environment. There are a variety of factors that affect the performance of heat sinks. Some of the factors include:                1. The surface area exposed to the working fluid;        2. The heat transfer coefficient of the surface;        3. The orientation of the exposed surface areas;        4. The thermal conductivity of the transfer surfaces; and        5. The aspect ratio of the product with respect to the heat source.        
As stated, some of the materials used for heat sinks are aluminum, zinc, and copper, with aluminum being the most common due to its reasonable cost/weight performance. The thermal resistance of the heat sink, Rba, changes based on the footprint area and how the orientation of the flat heat sink changes these values. For example, a horizontal aluminum heat sink with a thermal resistance of 32 C/Watt would have a footprint of roughly 2200 square mm. This would require a round heat sink with a diameter of 53 mm (2.1 inches).
Using equation 2 to calculate the total thermal resistance of a white Cree XLamp mounted on a Metal Core PCB with FR4 dielectric and a flat heat sink with a diameter of 53 mm, the following total resistance is obtained:Rja=8 C/W+7.3 C/W+32 C/W=47.3 C/W 
Assuming the maximum allowable ambient temperature is 85° C. and the power that must be dissipated is 1 W, will the junction temperature exceed the 145° C. maximum?Tj=Ta+Pd×Rja 
Placing values into this equation, the following is obtained:Tj=85 C+1 W×47.3 C/W=132.3 C 
In this situation, provided as the maximum power to be dissipated is 1 W or less, the maximum temperature at the junction will be less than the 145° C. maximum allowable.
In the 1980's, an industry to selectively plate three-dimensional plastic components was emerging. In the development stages of the industry, there were many techniques used by the various companies to selectively plate their products. Techniques such as hot stamping, pad printing, roller coating, film over-molding, film transfer, two shot molding, and three-dimensional masking were used.
Many of these techniques are complicated and require many steps to produce the end product, resulting in a very expensive and uneconomic process. Some of the other techniques that showed promise to be cost effective, had other limitations with respect to lead times for tooling or limitations on resolution.
Early in the new millennium, another process to image patterns onto three-dimensional interconnects emerged. This process was developed by LPKF Laser in Hanover Germany and was patented under U.S. Pat. No. 6,696,173. This process is unique in that it uses a group of plastics that have been doped with a catalyst that when exposed to a YAG laser beam will allow the plastic to be plated in the areas of exposure. In the past, fine pitch selective plating of single-shot plastics required that the part be molded, electroless-plated, electrophoretic-resist coated, exposed in three-dimensions, resist-developed, electroplated, stripped, and etched. Furthermore, this process could not be used for decorative parts because the entire surface of the product needed to be etched, making the surfaces non-cosmetic.
A second technique to manufacture products that has continued to be viable utilizes a two shot molding process. With this method, a material doped with a palladium catalyst is molded along with a non-doped material in a two shot molding operation. Wherever the doped material is exposed to the surface of the molded product, it will plate after it is etched with the appropriate etchant and then immersed into an electroless plating solution.
Each of these techniques has some advantages and limitations on how they can be used. The laser marking process is ideal when fine pitch patterns are used, when the plated area is relatively small, when changes may need to be made to the patterns, or when some surfaces must be decorative. It has some limitations in that it is a line of sight process. This makes plated through-holes more difficult to manufacture and any plating on surfaces that is not on a line of sight plane requires that the part be moved. Furthermore, the processing time under the laser is an additional cost that the two shot process does not have.
The two shot process has the advantage of being able to produce highly three-dimensional parts, plated through-holes are very simple to produce, and because the plated pattern is produced in the mold, it is a very cost effective technique to make products. The two shot process limitations include higher cost tooling, longer lead-time prototypes, and line/space limitations.